In this work a macroscopic approach to the energetics of open, multicomponent, unsteady-state systems with chemical reactions has been proposed. A set of uniform equations describing material and energy conservation and entropy production has been derived and then applied to some limited, idealized situations. Simple, ideal systems such as adiabatic rigid tanks, steady-flow adiabatic steam turbines, and a steady-flow continuous stirred-tank reactor have been analyzed from an energy conservation and entropy generation standpoint. The same methodology has then been used to characterize the thermodynamics of complex, open, biological systems. Quantitative determinations of the lost work rates have been made for a photosynthetic green leaf and, comparatively, for a forest-covered surface and a clear-cut surface inside the atmospheric boundary layer. Based on the results of the lost work analysis the most dissipative processes occurring in each of the systems analyzed have been determined;The thermodynamic analysis of separate systems has provided the tools for the thermodynamic evaluation of the interrelated and integrated subsystems of a controlled environment and life support system. A lost-work analysis or availability evaluation has been performed on the overall system to obtain an optimum of energy consumption and entropy generation by optimizing each separation device, process, and subsystem. The second-law efficiency has been determined for a plant module as a solar energy converter used to fulfill the requirements of the closed-loop, bioregenerative, life support system;In summary, methods of classical and non-equilibrium thermodynamics conjugated with transport phenomena, biochemistry, and reaction kinetics have been applied to understand complex, open, chemically reacting systems, and to quantitatively determine thermodynamic efficiencies and energy availabilities of processes occurring in systems with biological as well as physical-chemical changes.